Diabetic nephropathy is the single most common cause of end-stage renal failure, and is highly prevalent in both type 1 and type 2 diabetics. Hyperglycemia has been suggested to play a critical role in the pathogenesis of diabetic nephropathy. Transforming growth factor-2 (TGF-2) is a major effector leading to renal fibrosis in response to hyperglycemia. TGF-2 is expressed as a biologically inactive form and must be converted to its active form to elicit fibrogenic effects. Thrombospondin1 (TSP1) has been identified as the molecular regulator of TGF-2 activation under diabetic conditions. TSP1-mediated TGF-2 activation is involved in the development of experimental diabetic nephropathy and also plays a role in human diabetic nephropathy. Preliminary data demonstrate a novel inhibitory effect of cGMP-dependent protein kinase (PKG) on glucose-induced TSP1 expression and TSP1-mediated TGF-2 activation in mesangial cells, suggesting a therapeutic potential of PKG in diabetic nephropathy. The proposed studies will test whether genetically or pharmacologically increased PKG activity prevents the development of diabetic nephropathy through inhibition of TSP1-mediated TGF-2 activation in different diabetic mouse models. Specific Aim 1 will determine the mechanisms by which high glucose down-regulates PKG activity in kidney cells in vitro and its role in glucose-induced TSP1 expression, TGF-2 levels/activity, and extracellular matrix (ECM) production. Specific Aim 2 will determine whether increased PKG activity prevents the development of diabetic nephropathy in vivo. These studies will establish the significance of PKG in the development of diabetic nephropathy. Importantly, utilization of sildenafil (an inhibitor of cGMP specific phosphodiesterase 5) to increase PKG activity in the proposed studies is translational and relevant to human therapy. Sildenafil is a drug used to treat erectile dysfunction (ED, another diabetic complication in male patients) and pulmonary arterial hypertension (PAH) in clinic. The current proposed studies will test a novel potential application of sildenafil in the treatment of diabetic nephropathy in different diabetic mouse models. Therefore, these studies have significant clinical relevance, and will lead to the development of novel therapies to attenuate/treat diabetic nephropathy, a major complication of diabetes.